The present invention relates to optical attenuators.
In typical optical attenuators, as disclosed e.g. in EP-A-557542, an incoming optical beam is passed through an attenuation device with a defined transmission characteristic. Varying the transmission characteristic can control the amount of attenuation.
Polarization-compensated attenuators are known e.g. from U.S. Pat. No. 4,398,806, JP-A-07 294742, or JP-A-59 223401.
In some variable optical attenuators using attenuating devices with varying optical thickness for achieving varying attenuation properties, it has been found that polarization dependent loss (PDL) also increases with increasing attenuation. This is due to surface properties or internal structures of the attenuation device having preferred polarization orientations and resulting e.g. from the applied manufacturing process, such as surface or internal structures with preferred orientations resulting from e.g. metal deposition or surface texturing process.
It is an object of the present invention to provide an optical attenuator with improved polarization dependent attenuation characteristic. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to the present invention, an optical attenuator for attenuating an incoming optical beam comprises at least two attenuating devices, each having a polarization dependent attenuation characteristic with a preferred orientation. In other words, each attenuating device provides a polarization dependent attenuation characteristic, so that the transmission rate of each attenuating device depends on the state of polarization of the incident light. The polarization dependent attenuation characteristic is generally represented in a diagram showing the polarization dependency for the different states of polarization of the incident light. A preferred orientation for the polarization dependent attenuation means that the polarization dependent attenuation in a first direction (perpendicular to the propagation direction of the incident light beam) is smaller than the polarization dependent attenuation in a second direction perpendicular to the first direction (and to the propagation direction of the incident light beam). In case that the attenuating device has such preferred orientation for the polarization dependent attenuation, the polarization dependent attenuation characteristic has an elliptical shape with the first orientation as one main axis and the second orientation as the other main axis. A circular polarization dependent attenuation characteristic, however, means that the polarization dependent attenuation is exactly the same for all states of polarization of the incident light beam. This also means that there is no polarization dependent loss.
The at least two attenuating devices are arranged in a way that the preferred orientations of the polarization dependent attenuation characteristics substantially compensate each other, at least partially in defined locations. Thus, the polarization dependent attenuation characteristic for the at least two attenuating devices together becomes substantially circular, at least in the defined locations, so that there is substantially no polarization dependent loss. These defined locations of substantially no polarization dependent loss can be a line (e.g. a line of intersection) or an area, having any possible shaping dependent on the specific characteristics of the attenuating devices and their spatial arrangement with respect to each other.
In a preferred embodiment, each attenuating device provides an attenuation characteristic wherein the attenuation for the incident light beam varies in a defined direction. The preferred orientation of the polarization dependent attenuation characteristic of each attenuating device is substantially in the same direction as the varying attenuation characteristics or substantially perpendicular thereto. The attenuating devices are arranged that the respective directions of the varying attenuation characteristics are tilted with respect to each other, so that the preferred orientations of the polarization dependent attenuation characteristics substantially compensate each other resulting in a substantially compensated polarization dependent attenuation characteristic of the attenuating device at least along a defined direction of intersection.
In a preferred embodiment, the optical attenuator comprises n attenuating devices, with n=2, 3, 4, . . . . The n attenuating devices are substantially equal in characteristic having substantially the same polarization dependent attenuation characteristic. The directions of the preferred orientations of the polarization dependent attenuation characteristics are arranged tilted with respect to each other each in an angle of 180xc2x0/n. In the ideal case wherein the n attenuating devices have exactly the same characteristics, the elliptical polarization dependent attenuation characteristics of each of the n attenuating devices exactly superimpose to a circular polarization attenuation characteristic of the attenuating device.
In case that each of the n attenuating devices with substantially the same characteristic also have varying attenuation characteristics in defined orientations, it becomes clear that a substantially compensated polarization dependent attenuation characteristics can only be found along a crossing or intersection line or curve, where also the attenuation characteristics of the n attenuating devices substantially match, in case the polarization dependent attenuation characteristic is dependent on the varying attenuation characteristic.
One or more of the attenuating devices may also be tilted with respect to the incident optical beam in order to (further) reduce reflection effects.
It is clear that the attenuating devices need not necessarily be individual or physically separated devices, but can also be physically combined or attached with each other. The attenuating xe2x80x98devicesxe2x80x99 might also result from one manufacturing process, so that there might not be individual xe2x80x98devicesxe2x80x99. The term xe2x80x98devicexe2x80x99 as used here has to be interpreted in its broadest sense and is characterized more in the direction of common physical properties than in the direction of physically individual and separated units. In its broadest interpretation, the effect of substantial compensation of polarization dependent loss according to the present invention can also be achieved by providing multiple attenuation xe2x80x98devicesxe2x80x99 with different orientations for the polarization dependent attenuation characteristics in order to achieve a resulting polarization dependent attenuation characteristic substantially without orientation.
In a preferred embodiment, one or more of the attenuating devices are embodied as layers, such as metallic layers e.g. resulting from metal deposition process. The layers can be individual layers or xe2x80x98interwovenxe2x80x99 with each other, e.g. resulting from a common manufacturing process, so that the xe2x80x98layersxe2x80x99 are not physically separated from each other. One or more of such layers might be provided (e.g. coated) on a substrate.